The production of starch derivatives for use as industrial thickeners of high viscosity has a long history. Examples of such thickeners are carboxymethyl starch ethers as disclosed in U.S. Pat. No. 2,523,709 (Moe). Moe discloses the use of an alcohol to precipitate a carboxymethyl ether derivative of starch from aqueous solution and thereby free the starch derivative from water soluble electrolytes. Moe discloses that very high viscosities can be obtained with relatively dilute aqueous solutions of such a product.
U.S. Pat. No. 2,802,000 (Caldwell) discloses granular, cold-water-soluble starch ether derivatives that are derivatized as a granular, non-liquid mass. The amount of base used by Caldwell to etherify the starch is the equivalent of the acid formed by etherification or only a slight excess so that the product is neutral or slightly alkaline. The product is described as birefringent; i.e. the granules exhibit the characteristic crosses when examined with a polarizing microscope.
U.S. Pat. No. 2,853,484 (Lolkema et al.) discloses reacting granular starch with a polyfunctional esterifying and/or etherifying agent (e.g. in aqueous caustic) and then subjected to a cold swelling process wherein the starch is pre-gelatinized (then optionally precipitated with solvents or inorganic salts) and dried.
U.S. Pat. No. 4,452,978 (Eastman) discloses granular starch hydroxypropyl ethers having reduced pasting temperatures, i.e. the temperature at which an aqueous dispersion of the granular starch ether will gelatinize is reduced, by etherifying in a liquid medium comprising a low alkanol.
Despite the availability of methods such as those described above of derivatizing starch, granular starch-based thickeners having properties equivalent to commercially available cellulosic thickeners have not generally been commercially available. Thus, it would be desirable to develop a method capable of producing such a starch-based thickener.